The present invention relates to an optical communication system for transmitting a high-frequency analog signal such as a radio signal via an optical fiber.
This application is based on Japanese Patent Application No. 10-163561, filed Jun. 11, 1998 and Japanese Patent Application No. 10-309981, filed Oct. 30, 1998, the contents of which are incorporated herein by reference.
Along with recent development of mobile communication, expansion of radio communication service areas is required. To effectively utilize radio wave frequency resources and reduce cost of base station equipment, a scheme in which individual radio zones (cells) are made small, and instead, a number of radio zones are arranged at a high density has received a great deal of attention. This is called a picocell radio zone. To realize the picocell radio zone, a radio communication base station arrangement in which transmitting/receiving devices and transmitting/receiving stations are connected through optical fibers has been examined.
More specifically, a radio base station has transmitting/receiving stations and transmitting/receiving devices. A plurality of transmitting/receiving devices are prepared for one transmitting/receiving station. The output power from each transmitting/receiving device is made small for the picocell radio zones. The transmitting/receiving devices and the transmitting/receiving station are connected through optical fibers. The transmitting/receiving devices transmit signals received from a common transmitting/receiving station to subscribers and transmit signals received from subscribers to a common transmitting/receiving station. The output from each transmitting/receiving device is made small to reduce cost.
A transmitting/receiving device is mainly formed from an antenna section and placed in each cell. A transmitting/receiving station has a modem and a controller corresponding to the plurality of transmitting/receiving devices in the cells. Therefore, the transmitting/receiving station is also called a central control terminal station. An analog radio signal is optically transmitted through an optical fiber between the transmitting/receiving device and transmitting/receiving station. With this arrangement, each transmitting/receiving device can be made simple, compact, and low-cost, and one radio communication base station can provide a number of cells.
In this arrangement, the basic arrangement of a transmitting/receiving device includes only an antenna, and opto-electric and electro-optic conversion devices and does not depend on the data rate or modulation scheme of a radio signal. Therefore, even when the radio transmission scheme is changed, replacement of the transmitting/receiving device or change in constituent elements of the transmitting/receiving devices is unnecessary.
For the above optical analog transmission, an electro-optical converter (E/O converter) is required to convert an electrical signal into an optical signal. At the E/O converter, light intensity of a semiconductor laser element is modulated with a radio frequency signal. As the modulation scheme, a scheme of directly modulating a semiconductor laser element or a scheme using an external optical modulator is employed.
Advantages and disadvantages of these two schemes will be compared. In terms of modulation distortion characteristics, device scale, and device cost, the scheme of directly modulating a laser element is more advantageous.
However, the trend of technology obviously indicates carrier frequency shift to a higher frequency band, e.g., shift to the 2- to 5-[GHz] band as the capacity of a radio frequency signal increases. However, in a distributed feedback laser element (DFB-LD) as a representative laser element, the modulation frequency range with a relatively small modulation distortion is as low as 2 to 3 [GHz]. Therefore, direct modulation of a laser element using a radio frequency signal is becoming difficult.
As disclosed in, e.g., Japanese Patent Publication (KOKAI) No. 6-164427, a scheme (subcarrier transmission) of superposing an intermediate frequency subcarrier signal fIF modulated by a data signal on a pilot carrier signal fLO as a sinusoidal wave and optically transmitting the superposed analog signal from a transmitting/receiving station to a transmitting/receiving device has been proposed.
In the transmission scheme proposed in this prior art, the intermediate frequency subcarrier signal fIF is frequency-converted (up-converted) by a multiplied signal obtained by multiplying the received pilot carrier signal fLO on the transmitting/receiving device side, thereby obtaining a radio frequency signal. The laser element is used in a low frequency band with excellent modulation distortion characteristics, and the pilot carrier signal fLO is superposed on a frequency close to the intermediate frequency subcarrier signal fIF.
According to an embodiment described in the above prior art, a pilot carrier signal fLO having a frequency of 300 [MHz] is superposed near an intermediate frequency subcarrier signal fIF in the 200-[MHz] band, as shown in FIG. 1. In this scheme, on the transmitting device side, to ensure the noise characteristics of the radio frequency signal and increase the frequency stability, the CNRs (Carrier-to-Noise Ratios) of the received intermediate frequency subcarrier signal fIF and pilot carrier signal fLO must be high. That is, the noise level must be low.
However, in the frequency band near the pilot carrier signal fLO, the RIN (Relative Intensity Noise) increases. Therefore, when the pilot carrier signal fLO is arranged near the frequency band of the intermediate frequency subcarrier signal fIF, as in the prior art, the CNR decreases.
FIG. 2 shows the result of an experiment conducted by the present inventors. When the intermediate frequency subcarrier signal fIF is set at 500 [MHz] and the pilot carrier signal fLO is set at 550 [MHz], the RIN characteristics largely degrade in accordance with the optical modulation index of the pilot carrier signal fLO and, more particularly, at an optical modulation index of 15 [%] or more, as shown in FIG. 2. Therefore, the communication quality of a radio frequency signal greatly degrades.
Especially, when the optical modulation index of the pilot carrier signal fLO increases, degradation in RIN becomes conspicuous. Hence, a radio frequency signal generated by frequency-converting the intermediate frequency subcarrier signal fIF using the pilot carrier signal fLO contains a number of noise components and therefore has poor transmission characteristics. When a radio frequency signal containing a number of noise components is transmitted, the noise components adversely affect other radio frequency signals to impede radio communication. Solutions to this problem are required.
To cope with a shortage in channels due to the recent increase in number of subscribers or an increase in transmission rate, extensive studies have been made for radio communication using a frequency band higher than the conventional frequency band, e.g., millimeter waves or submillimeter waves. For this system as well, an arrangement for connecting transmitting/receiving devices and transmitting/receiving stations through optical fibers has been examined.
As a connection form using optical fibers, a PON (Passive Optical Network) is used. In the PON, as shown in FIGS. 3 and 4, a transmitting/receiving station 1 and a plurality of transmitting/receiving devices 2 are connected through optical fibers 4 in which a passive optical divider 3 is inserted. An optical signal transmitted from the transmitting/receiving station 1 to the optical fiber 4 is divided by the optical divider 3 inserted into the optical fiber 4, and distributed to the transmitting/receiving devices 2.
In the PON, a passive optical divider 3 is inserted midway along optical fibers 4 to accommodate the plurality of transmitting/receiving devices 2. Hence, the optical transmission/reception device of the transmitting/receiving station 1 and optical fibers 4 can be shared, and accordingly, the equipment can be made compact.
In the PON, an optical signal transmitted from the transmitting/receiving station 1 is divided, so the same signal reaches the plurality of transmitting/receiving devices 2. There is no problem when radio signals transmitted from the plurality of transmitting/receiving devices are completely equal. However, different transmitting/receiving devices 2 normally transmit different radio signals.
Conventionally, as shown in the spectrum arrangement in FIG. 5, an optical signal to be transmitted from the transmitting/receiving station to the transmitting/receiving device is frequency-multiplexed while changing the frequency of the intermediate frequency subcarrier signal fIF corresponding with each transmitting/receiving devices and sent (subcarrier multiplex transmission scheme). In this case, each transmitting/receiving device receives the optical signal, extracts a component to be transmitted from the self station, converts the component into a radio signal frequency, and transmits the signal from the antenna.
In the example shown in FIG. 5, the frequencies of the intermediate frequency signal are assigned at an appropriate interval and frequency-multiplexed: for example, a signal fIF1 to a transmitting/receiving device 2-1 is assigned near 100 [MHz], a signal fIF2 to a transmitting/receiving device 2-2 is assigned near 200 [MHz], and a signal fIF3 to a transmitting/receiving device 2-3 is assigned near 300 [MHz]. Therefore, if radio signals sent from the transmitting/receiving devices 2-1, . . . , 2-3 are in the 2 [GHz] band, the transmitting/receiving device 2-1 must up-convert the signal fIF1 by 1.9 [GHz], the transmitting/receiving device 2-2 must up-convert the signal fIF2 by 1.8 [GHz], and the transmitting/receiving device 2-3 must up-convert the signal fIF3 by 1.7 [GHz].
For a conventional radio system using optical subcarrier transmission, a method has been proposed in which not only the intermediate frequency subcarrier signal fIF but also the pilot carrier signal fLO as a signal for maintaining the frequency stability of the radio wave transmitted from the transmitting/receiving devices is transmitted, and each transmitting/receiving device frequency-converts (up-converts) the intermediate frequency subcarrier signal fIF using the pilot carrier signal fLO, as shown in FIG. 1.
As a consequence, when the frequencies of intermediate frequency subcarrier signals for the individual transmitting/receiving devices are different, as shown in FIG. 5, pilot carrier signals fLO for frequency conversion must be prepared for the respective intermediate frequency subcarrier signals fIF. Pilot carrier signals fLO corresponding to the number of intermediate frequency subcarrier signals multiplexed must be sent. These signals are multiplexed and sent in optical transmission.
As a result, the total number of signals including the pilot carrier signal fLO increases. Since the optical modulation index of the intermediate frequency subcarrier signals in optical transmission is shared by the pilot carrier signals fLO, the optical modulation index decreases to degrade the transmission quality.
In the radio system, when a plurality of radio base stations (transmitting/receiving devices) provide the same service, frequencies slightly different from each other in the same frequency band are sometimes used to prevent interference between signals from adjacent base stations.
For example, frequencies are separated at an interval of 100 [kHz] in the 2 [GHz] band. When such transmitting/receiving devices are accommodated through one fiber, a system in which subcarrier signals with different frequencies are multiplexed in the radio frequency band while only one pilot carrier signal is transmitted can be constructed.
In this case, however, each transmitting/receiving device 2 that has received the optical signal from the transmitting/receiving station 1 must select a signal to be used in the self station from signals arranged at an interval as small as 100 [kHz]. For this purpose, a very steep filter with high frequency stability is required, resulting in an increase in cost. In a radio system using a radio scheme other than frequency multiplexing, e.g., CDMA, signals transmitted from transmitting/receiving devices are in the same frequency band. Therefore, the method of transmitting only one pilot carrier signal fLO using a steep filter cannot be used.
To simplify the arrangement, an intermediate frequency signal to be used in the self station must be separated from intermediate frequency signals, which are multiplexed as subcarriers, using a simple filter, as described above. For this purpose, subcarriers are preferably multiplexed at a large frequency interval.
However, to do this, a plurality of pilot carrier signals fLO corresponding to the number of the intermediate signals fIF are necessary. To stabilize transmission quality, the optical modulation index should not be decreased. Sending more signals including a plurality of pilot carrier signals fLO means increasing the optical modulation index for the total signals. The amount of RIN corresponds to the optical modulation index. There is the effect of interference modulation as one of the others noise decreasing transmission quality. The effect of the interference modulation also corresponds to the number of signals and the optical modulation index. The plurality of pilot carriers inevitable degrades the data transmission quality.